Ion exchange resins are utilized in the production of high purity water. These resins are materials which are able to remove ionic impurities from water by a mechanism of selective ion exchange on a large number of active sites contained in the matrix of the resin. This process continues until such time as the active sites have been saturated with the ionic impurities. Commonly, at that time, the ion exchange resins are regenerated with highly concentrated ionic solutions (i.e., sulfuric acid, sodium hydroxide, sodium chloride, etc.,) to restore their capacity to remove the ionic impurities from high purity water. The highest purity water can be produced by utilizing a mixed bed of ion exchange resin beads. This is a combination of anion and cation exchange resin beads intimately intermixed.
Regeneration of anion resins in a high purity water treatment application is most commonly done with a strong caustic solution. Regeneration of cation resins in a high purity water treatment application is most commonly done with a strong acid solution.
To avoid improper regeneration of the cationic and anionic exchange sites, it is essential to separate the anion resins from the cation resins prior to the chemical regenerations. The most commonly practiced technique for separating anion and cation exchange resins is to fluidize, with water, the mixed resin bed in a cylindrical vessel to achieve a predetermined bed volume expansion. The anion resins, by virtue of their lower density, are preferentially transported to the upper regions of the cylindrical vessel. An additional result of this fluidization is to remove a portion of the insoluble impurities which may have been collected on the ion exchange bed while in service.
Once bed expansion has been effected, fluidization is discontinued and the expanded resin bed is allowed to settle. An interface normally forms between the anion and cation resin masses. Typically, the anion portion is drawn out of the separation vessel and transferred to a separate vessel where regeneration is performed. The remaining cation resins are subsequently regenerated in a separate vessel. Industrial experience with this method of separating, cleaning, and transferring resins has shown the process to result in considerable (1-5%) cross contamination of the anion and cation resins. That is, 1-5% of the cation resin beads end up in the anion regeneration vessel and vice versa. This results in 1-5% of the resin being regenerated into an improper ionic form for the production of high purity water. Additionally, in order to achieve adequate cleaning of the insoluble impurities from the resins, large volumes of water must be consumed (100-500 gallons per cubic foot of resin).
Efforts to overcome the resin cross contamination problem includes the following techniques: (1) Using a high concentration sodium hydroxide solution to "float" the anion resin beads away from the entrained cation resin beads; (2) Pretreating the separated anion resin beads with ammonium hydroxide to convert any entrained cation resin beads to the ammonia form (this practice is utilized in steam boiler designs that operate with ammoniated water to minimize system component corrosion); (3) Manufacturing ion exchange resins having a carefully controlled narrow resin bead size distribution which creates a large difference in terminal settling velocities between the anion and cation resins (this facilitates the hydraulic separability of the resins); (4) Including a small amount of inert resins (neither anionic or cationic) having a density intermediate that of the anion and cation resins in the resin mass (when conventional hydraulic separation is attempted, these inert resin beads form a thin layer between the anion and the cation resin masses. A specially designed fluidization vessel is utilized in this process. This vessel consists of a cylindrical top and a conical bottom. After initial fluidization, the cation resins are drawn off of the bottom of the conical section and passed by an inline conductivity monitor. This monitor is capable of measuring a conductivity difference between cation resin slurry, inert resin slurry, and anion resin slurry. Thus, when the end of the cation slurry phase is sensed by the conductivity monitor, flow is ceased and a resin separation has been effected).
All of the processes described above suffer from one or more of the following defects: (1) The resins are subjected to significant mechanical, thermal, and chemical stresses in the course of normal service, transfer, cleaning, and chemical regeneration. This results in some degree of physical breakdown of the resin beads. Thus, there is a continual degradation in the resin bead particle size distribution over a period of time. This brings about the production of quantities of fractured beads known in the trade as resin "fines". These fines create problems both in the operation of the ion exchange bed (i.e., increased differential pressure across the bed) and in the hydraulic separation of the mixed beads prior to regeneration. Unfortunately, a cation resin bead fragment can appear identical to a whole anion resin bead in terms of their hydraulic settling. Therefore, inadequate separation of the anion and cation resins can result. (2) The cleaning and separation of resins commonly depends upon the use of air and water to loosen and remove insoluble impurities from the ion exchange resins. Poor water flow distribution in the cylindrical vessels normally used for these processes results in pockets of little or no hydraulic activity. Thus, there are regions from which the insoluble impurities are not removed. To compensate for this inadequacy, the cleaning operation most often evolves into a multiple step process which consumes large volumes of water and requires a considerable amount of time. (3) As described above, poor water/resin contact can result in poor removal of insoluble impurities from the resins. This creates problems during the regeneration and subsequent rinsing of the resins. Clean resins are more receptive to both the chemical regenerants and water rinses than are dirty resins. (4) In many conventional resin transfer systems, the resin ratio and total volume of mixed resins must be carefully controlled in order to achieve any degree of separation. This is because a resin interface must be formed at precisely the correct height in the separation vessel in order to draw the anion resin beads away from the cation resin beads through the fixed point transfer piping. If this resin ratio is varied or the total resin volume is incorrect, severe cross contamination of the resins results. (5) In an effort to overcome some of the limitations inherent in some of the resin separation equipment, complex multiple step, multiple transfer processes have been developed by dedicated operators. While significant improvement is observed in some cases, the processes are extremely complicated and require a great deal of intuitive response on the part of the operator.
The present invention is directed to overcoming one or more of the problems as set forth above.